The control of genital herpes will require widespread use of effective vaccines. However, if herpes simplex virus (HSV) vaccines do not achieve sterilizing immunity (prevent virus replication at the entry site) the virus will establish latency, rendering the host potentially contagious during reactivation, and allowing continued transmission. While animal studies with a variety of vaccines show that immunization does not prevent virus replication in the genital mucosa following high titer challenge, a HSV type 2 glycoprotein D vaccine protected 39-46% of seronegative women against infection in a recent clinical Trial. Since much of the spread of genital herpes occurs during periods of asymptomatic shedding when relatively little virus is present, we believe that the protection resulted because immunization increased the virus inoculum required to infect the genital mucosa. In Aim 1 we will explore this hypothesis by determining the effect of immunization with the clinical study vaccine on the virus inoculum required to infect the genital mucosa in a mouse model. In Aim 2 we will again use the threshold of infection to measure efficacy and determine whether DNA prime glycoprotein boost improves protection compared to DNA or glycoprotein only immunization. These studies are relevant because an effective vaccine will need to induce T helper type 1 (Th1) responses in addition to antibody and DNA vaccine priming with protein boosting has been shown to increase Th1 responses compared to protein only immunization. While a vaccine that increases the threshold of infection will reduce the incidence of transmission, it will not provide universal protection. In Aim 3 we will use conditions that overcome protection from infection to examine the impact of immunization on the magnitude of latent infection and recurrent disease (both clinical recurrences and virus shedding into the genital tract). These studies will provide new information about the risks of transmission from immunized hosts who become infected. Taken together, the studies in this proposal will yield new information about the capacity of HSV vaccines to reduce the spread of genital herpes. These study designs may become standard for preclinical evaluation of HSV vaccines.